Technologies for integrating metals with metals, and metals with resins, are required in components in various industrial fields such as automobiles, domestic appliances, industrial machinery or the like, and are likewise required in the design and manufacture of structures. Numerous adhesives have been developed to meet these requirements, among which various excellent commercially available adhesives are in commercial use. For instance, adhesives that bring out their functionality at normal temperature, or upon heating, are used to integrally bond a metal and a synthetic resin. This method constitutes a standard bonding technique used at present.
Meanwhile, other bonding technologies that do not rely on adhesives have also been developed. Examples of such technologies include, for instance, methods for integrating light metals, such as magnesium, aluminum or alloys thereof, or ferrous alloys such as stainless steel, with high-strength engineering resins, without any intervening adhesive. Manufacturing technologies that have been developed and proposed include, for instance, methods that involve bonding simultaneously with injection or the like (hereafter, “injection bonding”), wherein a polybutylene terephthalate resin (hereafter, “PBT”) or a polyphenylene sulfide resin (hereafter, “PPS”) is injected and bonded with an aluminum alloy (for instance, Patent documents 1 and 2). In addition, the possibility of using these resins systems in injection bonding of magnesium alloys, copper alloys, titanium alloys and stainless steel has recently been demonstrated and proposed (Patent documents 3, 4, 5 and 6).
These inventions, all of which stem from the same inventors, derive from a simple bonding (fixing) theory, namely an “NMT” theoretical hypothesis relating to injection bonding of aluminum alloys, and a “new NMT” theoretical hypothesis relating to injection bonding of all metal alloys. The theoretical hypothesis “new NMT”, having a wider reach, and advanced by one of the inventors (Ando), posits the following. To bring out the strong bonding strength of injection bonding, it is necessary that both the metal alloy and the injection resin meet several conditions. Among these, the metal must meet the three conditions below. In condition (1), the chemically etched metal alloy has preferably a rough surface (surface roughness) exhibiting a period of 1 to 10 μm (spacing between peaks or spacing between valleys) wherein the peak-valley height difference is about half the spacing, i.e. about 0.5 to 5 μm.
Such roughness (surface roughness) cannot be totally achieved in practice through chemical reactions. Condition (1) is deemed to be satisfied when surface roughness, as measured using a surface roughness analyzer, yields a roughness curve with a maximum height difference ranging from 0.2 to 5 μm for textures of irregular period ranging from 0.2 to 20 μm, or when the mean width of elements (roughness curve average length) (RSm) ranges from 0.8 to 10 μm and the maximum height roughness (maximum height of profile) (Rz) ranges from 0.2 to 5 μm in accordance with Japanese Industrial Standards (JIS B 0601:2001(ISO 4287)), based on scanning analysis using a scanning probe microscope. The inventors denominate a roughness thus defined as “surface of micron-scale roughness”. As condition (2), the above large irregular surface, strictly speaking the inner wall face of the recesses thereof, has a fine irregular surface of a period not smaller than 10 nm, preferably a period of 50 nm. As the last condition (3), the surface that constitutes the above fine irregular surface is a ceramic substance, specifically a metal oxide layer thicker than a native oxide layer, or a deliberately created metal phosphate layer. This hard-substance layer, moreover, is preferably a thin layer having a thickness ranging from several nm to several tens of nm.
As regards the resin conditions, suitable resins that can be used are hard crystalline resins having a slow crystallization rate upon rapid cooling, for instance through compounding with other polymers that are appropriate for the resin. In practice there can be used resin compositions in which a crystalline resin such as PBT or PPS is compounded with other appropriate polymers, as well as with glass fibers and the like. These resin compositions can be injection-bonded using ordinary injection molding machines and injection molding molds. The injection bonding process is explained next according to the “new NMT” hypothesis of the inventors. The injected molten resin is generally led into an injection molding mold at a temperature lower than the melting point of the resin by about 150° C. The molten resin is found to cool within flow channels of the molten resin, such as sprues, runners and the like, down to a temperature lower than the melting point. It will be appreciated that no immediate phase change to solid occurs in zero time, through crystallization when the molten crystalline resin is cooled rapidly, even at or below the melting point of the molten resin.
In effect, the molten resin persists in a molten, supercooled state for a very short time also at or below the melting point. The duration of this supercooling has been successfully prolonged somewhat in PBT and PPS through some special compounding, as described above. This phenomenon can be exploited to cause the molten resin to penetrate into large, micron-scale recesses on the surface of the metal, before the abrupt rise in viscosity that accompanies the generation of large amounts of micro-crystals. After having penetrated into the recesses, the molten resin goes on cooling, whereby the number of micro-crystals increases dramatically, causing viscosity to rise abruptly. The size and shape of the recesses determine whether the molten resin can penetrate or not all the way into the recesses. Experimental results have revealed that, irrespective of the type of metal, the molten resin can penetrate thoroughly into recesses having a diameter not smaller than 1 μm and having a depth of 0.5 to 5 μm. When the inner wall faces of the recesses, have also a rough surface, as evidenced in the above-described microscopic observations (micrographs), the resin penetrates partly also into the crevices of these ultra-fine irregularities. As a result, the resin catches onto the irregularities and is difficult to pull away when a pulling force acts from the resin side.
Such a rough surface affords an effective spike-like catching when the surface is that of a high-hardness metal oxide. If the period of the irregularities is 10 μm or greater, the bonding force weakens for the evident reasons below. In the case of dimple-like recess aggregates, for instance, the number of dimples per surface area decreases as the diameter of the recesses becomes larger. The larger the recesses are, the smaller the catching effect of the above-mentioned spikes. Although bonding per se is a question of the resin component and the surface of the metal alloy, adding reinforcing fibers or an inorganic filler to the resin composition allows bringing the coefficient of linear expansion of the resin as a whole closer to that of the metal alloy. This allows preserving easily the bonding force after bonding. Composites obtained through injection bonding of a crystalline resin such as a PBT or PPS resin, with a magnesium alloy, copper alloy, titanium alloy, stainless steel or the like, in accordance with the above hypothesis, are strong integrated products, having a shear fracture strength of 200 to 300 kgf/cm2 (about 20 to 30 N/mm2=20 to 30 MPa).
The present inventors believe the “New NMT” theory to be true as borne out in injection bonding of numerous metal alloys. The advocated hypothesis, which is based on inferences relating to fundamental aspects of polymer physical chemistry, must be reviewed by many chemists and scientists. For instance, although we have freely argued about the molten crystalline resin upon rapid cooling, the question of whether the crystallization rate really drops has not been discussed heretofore from the standpoint of polymer physics. Thus, although we believe the hypothesis to be correct, the latter has not been proved true outright. Specifically, high-rate reactions at high temperature and under high pressure cannot be observed directly. The hypothesis, moreover, postulates a purely physical anchor effect underlying bonding, which deviates somewhat from conventional knowledge. Most monographs and the like concerned with adhesion and authored by specialists ordinarily ascribe chemical factors to the causes underlying adhesive forces.
Owing to the experimental difficulties involved, the inventors gave up on validating their hypothesis through direct experimentation, and decided on a reverse approach. Specifically, the inventors assumed that the “new NMT” theoretical hypothesis can be applied also to adhesive bonding, and set out to study whether high-performance adhesive phenomena can be proved by a similar theory. That is, the inventors decided to ascertain whether non-conventional bonded systems can be discovered based only on the surface state of adherend materials, and using commercially available general-purpose epoxy adhesives.
Remarkable developments have been achieved in bonding by way of adhesives. In particular, high-technology adhesives are being used in aircraft assembly. In these technologies, bonding is accomplished using high-performance adhesives, following a surface treatment in which a metal alloy is imparted corrosion resistance and microscopic texture. On closer inspection, however, metal surface treatment methods such as phosphoric acid treatment, chromate treatment and anodization rely still on staple treatment methods developed 40 or more years ago, and it seems as though no new developments have come along in recent years. As regards the development of adhesives themselves, mass production of instant adhesives took off several decades ago, and no new breakthroughs have been achieved since the landmark introduction of second-generation acrylic adhesives. From the viewpoint of adhesion theory as well, and although the inventors may not be aware of the very latest academic trends, the chemical and physical explanations jointly proffered in the commercially available monographs and the like appear to us lacking in clarity and also in ideas that may lead to further developments.
Fortunately, it is possible to use nowadays, freely and inexpensively, electron microscopes having resolutions of several nm. The inventors have discussed their “NMT” and “new NMT” hypotheses relating to injection bonding on the basis of observations of such high-resolution micrographs. As a result of the observations, the inventors eventually proposed the above-mentioned hypothesis, thoroughly based on the anchor effect. Therefore, we expected novel phenomena to be observed as a result of working on adhesion theory, in terms of adhesive bonding, by emphasizing the physical aspects. Titanium alloys have special characteristics in that, for instance, their strength is comparable to that of steels, but with a specific weight of about 4.5. Among metals in practical use, titanium alloys are thus both high-strength but lightweight metals. Titanium alloys, moreover, are resistant to salt water, and elicit very low irritability on the skin and living organisms. For this reason, titanium alloys are used not only as materials in the above-described mobile equipment such as automobiles, but also as medical materials implanted in vivo, in the form of prosthetic arms, legs, teeth, joints and the like.
The inventors began working on prototypes of outer casings for notebook computers and the like made of a PPS resin, and on plates of, for instance, titanium alloys of JIS grade I pure titanium, using injection bonding (Patent document 5) already developed. We also looked into the possibility of manufacturing similar cases for mobile electronic devices, structural members for mobile equipment, outer panel members and other parts, using now adhesives instead of injection bonding. In particular, carbon fiber reinforced plastics (hereafter, “CFRP”) have the highest tensile strength among structural materials, including metals, and are ultra-lightweight, having a specific weight of 1.6 to 1.7. Lightweight and strong composite parts or structures can thus be potentially manufactured by combining composite CFRPs and lightweight titanium alloys having also light weight and high strength. CFRP prepregs are fabrics or aggregates of carbon fibers impregnated with an uncured epoxy resin. Composite formation and integration simultaneous with curing can be made simple by tweaking the affinity of CFRP prepregs and an epoxy adhesive coated on the metal.
To manufacture integrated products, therefore, we felt that first of all it was necessary to conduct diligent research and development on how to improve and stabilize bonding forces between titanium alloys and epoxy adhesives. Thus, we endeavored to develop a method that affords solid bonding with fiber-reinforced plastics (hereafter, “FRPs”), in particular CFRPs, by focusing on the development of surface treatment techniques for titanium alloys.
Patent document 1: WO 03/064150 A1
Patent document 2: WO 2004/041532 A1
Patent document 3: PCT/JP 2007/073526 (WO 2008/069252 A1)
Patent document 4: PCT/JP 2007/070205 (WO 2008/047811 A1)
Patent document 5: PCT/JP 2007/074749 (WO 2008/078714 A1)
Patent document 6: PCT/JP 2007/075287 (WO 2008/081933 A1)